The human eye comprises a roughly spherical organ having essentially three distinct layers of tissue divided into three basic chambers. The tough outer sclerotic coat serves as a protective barrier for the eye, and forms the transparent cornea through which light passes into the eye. The sclerotic coat is composed of dense collagenous tissue. The middle choroid coat forms the iris, a diaphragm that controls the amount of light admitted into the interior of the eye through the pupil. The transparent crystalline lens is immediately posterior to the iris, and is held in place by zonular fibers attached to ciliary processes surrounding the crystalline lens. The zonular fibers collectively culminate in the suspensory ligament of the lens. The region between the cornea and crystalline lens is denoted the anterior chamber of the eye, whereas the gap created between portions of the crystalline lens and iris is known as the posterior chamber. Ciliary processes generate aqueous humor, which fills the anterior chamber and posterior chamber. Aqueous humor provides for nutrient and metabolic exchange between the avascular cornea, crystalline lens, and iris. The posterior pole of the crystalline lens abuts the hyaloid fossa of the posterior vitreous chamber of the eye.
Accommodation is the process of changing the focus of the eye between near and distant objects. Accommodation is achieved by constricting and relaxing the ciliary muscle connected to the crystalline lens via the zonular ligament. Such movement by the ciliary muscle serves to shape the crystalline lens to the appropriate optical configuration for focussing light rays from these objects onto the inner coat of the eye, structurally known as the retina.
The crystalline lens is a biconvex body, having an anterior convexity less steep and of a greater radius of curvature than its more parabolic posterior convexity. The lens is composed of elongated, prismatic cells known as lens fibers, which are tightly packed to form lamellar structures. Intracellular granular crystallins within the lens fibers confer transparent and refractive characteristics upon the lens. Lens fiber structure and composition varies within the lens, such that a firm central nucleus may be distinguished from a softer surrounding cortex. The entire lens is encompassed by the lens capsule (capsula lentis), a basement membrane into which the zonular fibers are inserted. The elastic lens capsule is composed of collagen fibers, glycosaminoglycans and glycoproteins. Due to its elastic properties, the lens capsule can stretch substantially in circumference without tearing.
A variety of disorders are known to impair or destroy normal function of the eye, including disorders of the lens, such as cataracts and presbyopia. Cataracts arise from progressive clouding of the crystalline lens, which, if left untreated, eventually scatters and obscures light rays from focussing on the retina. Historically, cataracts were surgically treated by either intracapsular removal of the entire lens structure, including the outer lens capsule and the inner crystalline lens matter, or extracapsular removal of the central portion of the anterior capsule and the crystalline lens matter, leaving in place the posterior lens capsule, the latter treatment of which is known in the art as the ECCE procedure. These procedures are prone to complications, such as retinal detachment, and, in the case of extracapsular cataract extraction, opacification of the posterior capsule.
As the crystalline lens ages, it becomes less flexible and gradually loses the ability to change shape in response to the contraction of the ciliary muscle. This is manifested in humans as a gradual loss of the ability to increase the refracting power of the lens and consequent reduction in the ability to focus the eye on nearer objects. This loss of focusing ability is called presbyopia.
Presbyopia is most commonly treated by bifocal and other multifocal spectacles and contact lenses. These devices typically comprise discrete zones of differing optical power through which the wearer's gaze is selectively directed to obtain a clear image for a given viewing distance. Common disadvantages are that these refracting powers are static and they are restricted to specific gaze positions. Other types of multifocal contact lenses and intraocular lenses are designed to simultaneously produce images through multiple refractive zones. A common disadvantage of these devices is that the contrast of the focused part of the image is reduced by the poorly focussed portions of the image.
Recently developed lens refilling procedures may reduce the incidence of many complications associated with traditional crystalline lens extraction and replacement treatment modalities. One such procedure is disclosed in U.S. Pat. No. 4,002,169, in which a rotary masticating tool is introduced into the lens structure via an inserted hollow needle. The capsular tissue contents, including the cataract, lens cortex and lens nucleus, are physically liquefied and then withdrawn from the lens capsule via suction through the needle. Such a process leaves the lens capsule intact as a capsular bag within the posterior chamber.
Often, a chemical treatment or sonication (phacoemulsification) is preferred over physical mastication for liquefying the lens. Following suction removal of the liquefied crystalline lens, the capsular bag may be flushed to remove remaining debris and then refilled with a molded synthetic lens. See, for example, the processes disclosed in U.S. Pat. No. 5,674,282.
Intraocular lenses may comprise relatively hard materials, relatively soft materials, or a combination of both types of materials. For example, methyl methacrylates, polysulfones or other relatively hard, biologically inert optical materials may be used alone, or in combination with softer biologically inert silicones, hydrogels or semi-rigid thermolabile materials. While these lenses restore optical transparency and, if comprising the appropriate optical power, can cause the eye to produce clear images of distant objects, their optical power is static and therefore can not change focal length as the viewing distances change.
Other intraocular lenses have multiple zones of varying curvature to provide a range of static refracting powers. These are generally referred to as bifocal or multifocal intraocular lenses. The retinal image produced by such lenses contains focussed and defocussed images. The defocussed images reduce the contrast of the focused image and therefore reduce the ability of the eye to obtain good resolution for all viewing distances.
A new lens may be created in situ in the lens capsule with a filler material having the appropriate characteristics to mimic the function of the natural crystalline lens. Many ophthalmic procedures designed to restore accommodation of the eye, such as lens refilling procedures for the correction of presbyopia and cataracts, rely on the replacement of endogenous lens matrix material with a transparent material of similar consistency and index of refraction and spectra.
Some of the preferred materials for filling the capsular bag comprise UV-curable polymers that require exposure to ultraviolet light to induce crosslinking. Such crosslinking typically requires creating two openings in the wall of the eye via bimanual surgery, which occupies both hands of the ophthalmic surgeon. Alternatively, crosslinking may be effected through the cornea, but such procedures may damage corneal tissues.
U.S. Pat. No. 5,391,590 discloses compositions useful as injectable intraocular lens material. Examples of polymerizable formulations include one or more polyorganosiloxanes having a vinyl functionality, a silicon-bonded hydride group, and the like. Such compositions may comprise soft, fast curing, low temperature vulcanization silicone gels capable of in situ polymerization, within the capsular bag. High molecular weight, high viscosity silicone precursor fluids are preferred, as they are less likely to leak from the injection site prior to polymerization. Such high viscosity materials only require a low cross-linking density to achieve an elastic modulus similar to a human crystalline lens. However, a reduced cross-linking density of these polymers results in an unacceptable gummy product having low resilience.
Certain low viscosity, low molecular weight fluids have desirable properties upon cure for injectable ocular lenses, but readily leak from the injection site. Upon curing of leaked gel, a bump may form on the surface of a refilled capsule. Such bumps are known to irritate the iris and mediate corneal edema. In an attempt to overcome this limitation, suitable low molecular weight fluids may be pre-cured to induce polymerization prior to injection into the lens capsular bag. Injection of such partially polymerized materials through a cannula may cause shear stress, which results in rough areas of the polymerized material that impair the function of the synthetic lens. Additionally, pre-cured polymer materials typically must be injected shortly after initiating crosslinking to prevent disadvantageous over-curing and reduced flow through the cannula.
Typically, the capsular bag tends to under fill unless very high density materials, such as gels having a viscosity of greater than 4×106 cS, are used. As mentioned hereinabove, viscous liquids and gels having less than 1×106 cS viscosity or soft gels are injected and introduced into the capsular bag, such fluids often leak from the bag. Leakage of such materials into the anterior chamber of the eye may cause a number of ocular problems, and endanger delicate ocular structures. For example, intraocular inflammation may be spurred by a foreign body reaction of the eye in response to the leaked material. Additionally, leaching of non-endogenous liquids or gels from the capsular bag may cause glaucoma, due to blockades of trabeculae and associated increases in intraocular pressure due to increased volumes of aqueous humor. Undesirable conditions, such as interference with motion of the iris and impairment of the optics of the eye due to glare are also known to occur upon escape of viscous liquids and gels introduced to the capsular bag.
Similarly, cataract surgery may require the introduction of a chemical agent to liquefy nuclear matter, and/or injection of a chemical or pharmacological agent to kill lens epithelial cells or impair their replication. Leakage of antimitotic compounds or hypoosmolar solutions destroys healthy, non-regenerative corneal endothelial and retinal cells of the eye, as opposed to the intended hyperproliferative lens epithelium.
An anterior capsulotomy, specifically a capsulorhexis, is typically used to reduce some of the procedural and post-operative complications associated with extracapsular and lens refilling protocols. A continuous tear capsulorhexis involves preparing a circular or round capsulotomy in the anterior lens capsule. In cases of ECCE, and peripherally in the case of lens refilling, and removing the essentially circular portion of the anterior capsule delineated by the continuous tear line, a continuous tear capsulorhexis forms an essentially circular tear line substantially coaxial with the lens axis.
Preferably, the capsulotomy is positioned within the zonule-free area of the anterior lens capsule. This type of capsulotomy forms a circular opening in the anterior lens capsule, through which cataractous lens matrix may be extracted by, for example, phacoemulsification and aspiration. What remains is a capsular bag having an elastic posterior capsule, an anterior capsular remnant about the anterior capsulotomy, and an annular capsular bag sulcus between the anterior capsule remnant and the outer circumference of the posterior capsule. Thus, the capsular bag remains attached to the surrounding ciliary muscle of the eye via the zonules, and is responsive to ciliary contraction and relaxation during accommodation.
Although continuous tear capsulorhexis is designed to provide an anterior capsule remnant or rim having a relatively smooth, continuous inner edge abutting the capsulotomy, the anterior rim is sometimes torn, radially sliced, or nicked during this procedure. Such damage to the anterior rim leaves the rim vulnerable to tearing radially when the rim is stressed, particularly upon insertion of instruments for manipulating the capsular lens matrix. Tearing of the lens capsule during capsulorhexis increases the likelihood of untoward leakage of materials injected into the evacuated capsular bag during lens refilling. To reduce the risk of such tearing, a deep anterior chamber is maintained throughout the surgery using a balanced salt solution or a viscoelastic material to fill the chamber. However, tears may arise despite taking such precautionary measures.
In an effort to address some of these ongoing problems in ophthalmic surgery, Nishi et al. (Graefe's Arch Clin Exp Ophthamol (1990) 228:582–588) developed a new disk for small-incision surgery, which also serves to seal the capsular opening. Following a circular mini-capsulorhexis and phacoemulsification procedures, an acrylamide synthetic disk larger than the capsular opening is inserted into the opening. After injecting a visco-elastic material into the capsular bag and anterior chamber of the eye, the disk is inserted into the anterior chamber. The disk is then manipulated such that the disk is choked by the entire capsular margin along its circumference, thereby fixing the disk in place of the missing portion of anterior capsule. Since the disk seals the opening of the lens capsule, the lens capsular bag is capable of refilling. Thus, a replacement material, polyacrylamide gel, is injected into the capsular bag to expand the bag. Although generally successful, certain drawbacks exist with this process, including expansion of the capsulorhexis opening during filling, causing intraoperative leakage. Moreover, Nishi et al. reported difficulties achieving a reproducible, centrally positioned circular capsulorhexis of an appropriate size for securely holding the inserted synthetic lens in the capsular bag. Furthermore, patients receiving such implantation developed capsular bag distention causing blurred vision.
Nishi, et al. (Arch Ophthalmol (1998) 116(10):1358–1361) recently devised a tube having a flange made to fit a surgically generated capsulorhexis opening in a patient's capsular bag. This tube is actually an implant since it is permanently bonded to the edges of the capsulorhexis with a silicone-based adhesive. Thereafter, a clear gel is injected through the tube via a 30 gauge stainless steel cannula. After filling the capsular bag, an adhesive within the tube seals the tube. The tube is then cut to remove excess length, although the remaining tube slightly protrudes from the bag into the anterior chamber of the eye. The protrusion of this implant may mechanically interfere with motion of the iris, impairing pupillary opening and closing. Contact of the inner surface of the iris causes drag, which may interfere with ocular accommodation. In addition, the protruding tube may scratch the corneal endothelium upon rubbing of the patient's eye containing the implant. Such implants are susceptible to biocompatibility problems, and may cause severe inflammatory reactions within the eye.
The methods described above attempt to significantly restore the accommodative capacity of the eye. However, such methods have not been completely successful. A lens system and surgical method or protocol for restoring accommodation to an eye would be particularly advantageous.